1. Field of the Invention
The present invention relates to an apparatus and method for heating an optical fiber preform in the process of fabricating an optical fiber, such as, for instance, sintering a porous optical fiber preform, collapsing a circular tube, jacketing a quartz tube, or extending or drawing an optical fiber preform. When the aforementioned apparatus and method of the present invention are used for heating the optical fiber preform, the optical fiber preform can be sintered, collapsed, jacketed, or extended or drawn while maintaining a uniform temperature distribution over an entire circumference of the optical fiber preform. Thereby, the circularity of the obtained optical fiber preform or an optical fiber may be improved.
2. Description of the Related Art
An optical fiber is fabricated by melting and drawing an optical fiber preform by use of, for instance, an optical fiber preform-heating furnace. Such an optical fiber preform-heating furnace 51 is, as shown in a longitudinal cross sectional view of FIG. 3, configured generally in such that a furnace core tube 52 into which the optical fiber preform 57 is supplied from an upper end side thereof, a heater 53 that surrounds the furnace core tube 52 and has a pair of electrode portions 532 each of which is disposed at an upper side of the heater and faces to each other, and a heater-surrounding heat insulator 54 that surrounds the heater 53 are accommodated in a stainless body 55 that is structured in a water-cooling jacket.
When an optical fiber (a linear product) 58 is fabricated by use of a generally used optical fiber preform-heating furnace 51 according to the aforementioned conventional example 1, a voltage is applied to a pair of electrode portions 532 from a not shown power supply to heat the heater 53. While thereby heating and melting the optical fiber preform 57 that is supplied into the furnace core tube 52, the optical fiber 58 is drawn from a lower end side of the furnace core tube 52. This optical fiber preform-heating furnace 51 can be naturally used also for, for instance, sintering a porous preform, collapsing a circular tube, jacketing a quartz tube, and drawing an optical fiber preform.
As a parameter that shows how a cross section of the optical fiber, a linear product, deviates from a perfect circle, there is noncircularity (%) that is expressed by ((major axis−minor axis)/an average diameter)×100. When an optical fiber has the noncircularity that is near zero, it can be said excellent in quality. That is, as the noncircularity of the optical fiber deviates largely from a perfect circle, a diameter of a hole that is used for fitting the optical fiber and formed in a ferrule that is used to align an optical connector has to be made larger. Accordingly, an axis line of the hole and that of the optical fiber deviate largely, resulting in a larger optical connection loss of the optical fiber. Likewise, in the case of the optical fiber being pressed onto a V groove formed in an alignment block and these being connected together, when a radius of the optical fiber of a portion that comes into contact with the V groove fluctuates and deviates from a perfect circle, an axis misalignment at the time of connection and optical connection loss are caused. It is not preferable too.
In the case of the optical fiber preform-heating furnace 51 according to the aforementioned conventional example 1 that has a general configuration, a heat current escapes from the heater 53 through the pair of electrode portions 532. Thereby, low temperature portions are locally generated in the heater 53. As a result, the uniformity of the temperature distribution in a circumferential direction of the furnace core tube 52 becomes insufficient and only the optical fiber 58 that has the noncircularity of 1% or more can be drawn. That is, there is a problem in that the optical fiber 58 having a small noncircularity is difficult to fabricate.
An optical fiber drawing furnace by which a temperature distribution uniformity in a circumferential direction of a furnace core tube is attempted to improve has been proposed in, for instance, Japanese Unexamined Patent Application Publication No. HEI 9-71433. In the following, the optical fiber drawing furnace according to this conventional example 2 will be described with reference to FIG. 4 that is a plan sectional view thereof, FIG. 5a that is a perspective view showing an appearance of a heater thereof and FIG. 5b that is a perspective view showing an appearance of the other heater thereof with the same names and the same reference numerals as the same patent publication.
That is, the optical fiber drawing furnace according to the conventional example 2 is attempted to overcome a problem accompanying the optical fiber preform 15 due to the nonuniformity of the temperature distribution in a circumferential direction of the optical fiber preform 15 that is heated by the circular heater 18, the optical fiber, a linear product, cannot be drawn from the optical fiber drawing furnace with smaller noncircularity. More specifically, a furnace core tube 13 therein the optical fiber preform 15 is supplied; a heater 18 that is arranged outside of the furnace core tube 13 and, as shown in FIG. 5a or FIG. 5b, formed in the shape in which slits are alternately disposed from both ends of a cylindrical body; and electrode connection portions 22 through 25 that protrude outwardly from the heater 18 and connect through a plurality of electrode portions 29 and 30 to a power supply 33. In the aforementioned optical fiber drawing furnace, as a means for equalizing a temperature distribution in a circumferential direction of the aforementioned heater 18, the number of the electrode connection portions 22 to 25 is made so as to outnumber that of the electrode portions 29 and 30.
According to the optical fiber drawing furnace of the aforementioned conventional example 2, as mentioned above, in order to equalize the temperature distribution in a circumferential direction of the furnace core tube 13 (the temperature distribution in a circumferential direction of the optical fiber preform 15), an equalizing means is provided so that the number of the electrode connection portions 22 through 25 may outnumber that of the electrode portions 29 and 30.
This equalizing means is effective in equalizing the temperature distribution in a circumferential direction of the furnace core tube (the temperature distribution in a circumferential direction of the optical fiber preform) and allows fabricating an excellent quality optical fiber that has a sectional shape of which noncircularity is less than 0.2%, that is, of a near perfect circle. Accordingly, the optical fiber drawing furnace according to the conventional example 2 is superior to the optical fiber preform-heating furnace according to the conventional example 1 according to a general configuration.
However, in the case of the optical fiber drawing furnace according to the conventional example 2, since a structure from the heater to the electrodes portions become complicated, not only the optical fiber drawing furnace becomes high in cost but also it takes a longer time to disassemble, to inspect and cleanse, and to reassemble. Accordingly, there still remains a problem to be overcome in that this optical fiber drawing furnace is disadvantageous in capacity utilization as well as in running cost.
That is, in spite of the optical fiber preform-heating furnace is simple in its structure, and allows fabricating an optical fiber having the same noncircularity as that of the optical fiber drawing furnace according to the aforementioned conventional example 2 is desirable.